Such solar collectors are now being used in all continents, and their design has remained virtually the same since the first of these collectors were used in Florida decades ago and is diagrammatically shown in FIG. 1. The most essential component is a sheet-metal element, which is irradiated by the sun at an angle which should approximate a right angle as closely as possible and which on the side facing the sun has been blackened by any of various possible methods in such a manner that this side has a maximum absorption capacity for visible light and is heated as a result of such absorption.
This sheet-metal element 1 has a high thermal conductivity and conducts the sensible solar heat to metal tubes 2, which are joined to the sheet-metal element and are traversed by a heat-transfer liquid 3, which consists preferably of beneficiated water and subsequently delivers its heat content, possibly through heat exchangers, for purposes such as room heating, water heating, the operation of absorption refrigerators for household and room cooling, the temperature control of swimming pools and the storage of such heat in highly heat-insulated storage tanks.
A given solar radiation causes the absorber to assume a maximum temperature T.sub.a, which reaches an equilibrium when the rate at which solar energy is absorbed equals the total rate at which heat is delivered by heat conduction, convection, radiation and by withdrawal of liquid.
To minimize the abovementioned dissipative effects, the absorber 1, 2 is usually enclosed in a box 4, which is provided with sufficiently thick heat-insulating interlayers 5 consisting of rock wool or porous plastics or ceramic materials and reduces the loss of heat from the absorber 1 and hot water tubes 2 to the environment.
The dissipation of heat from the side facing the sun cannot be decreased by such a simple expedient because the abovementioned heat insulators, which are opaque, cannot be used. On the side facing the sun, the box must be covered with one or more glass panes or sheets of plastic (synthetic-resin) material 6, which absorb infrared radiation to reduce the dissipation of heat by radiation. However, they reflect and absorb visible light and thus effect an attenuation in this part of the spectrum.
In accordance with the known Stefan-Boltzmann law, the heat radiation from the absorber equals E = .sigma. .times. T.sub.a.sup.4 and in accordance with Wien's displacement law has an energy maximum at a wavelength .lambda..sub.ma = 2900/T.sub.a so that in a definite case, in which T.sub.a = 400.degree. K, .lambda..sub.max = 7.2 .mu.m in the near-infrared range. A covering of the side facing the sun with glass panes or synthetic-resin material sheeting cannot result in a satisfactory insulation against a dissipation of heat by conduction and convection because such coverings are rather thin.
Whereas the absorption of solar radiation will be reduced if the radiation is caused to fall at an oblique angle on the absorber plane 1 in that the same is continually adjusted in dependence on the position of the sun, such an arrangement is complicated and expensive in practice. For this reason the planar absorbers are fixed with such an orientation that a maximum mean value of the absorption of radiation between sunrise and sunset is obtained in accordance with the known laws of trigonometry. Calculation shows that this requirement will be met if the absorber plate is inclined toward the south (on the northern hemisphere) at an angle which exceeds the geographical latitude by about 10.degree.. For this purpose, a support 7 is provided.
It will be appreciated and can be quantitatively derived by calculation from the heat balance that the maximum temperature T.sub.a assumed by the solar collector and its maximum heat output are proportional to the ratio of solar power input to heat delivery rate. At an absorber temperature T.sub.a = 393.degree. K (=120.degree. C) the dissipation of heat by radiation having a constant o = 5.73 .times. 10.sup.-11 (kW/m.sup.2 .times.T.sup.4) will reach the ideal value 1.35 kW/m.sup.2 of the solar constant and must not be neglected in view of the dissipation of heat by convection and conduction, which is proportional to the temperature difference T.sub.a -T.sub.u (T.sub.u = absolute ambient temperature).
As a quantitative numerical example it is stated that a perfected thermal insulation of a Florida-type solar collector as shown in FIG. 1 provides for a measured mean field coefficient of heat transfer .alpha. = 2.2 kcal/m.sup.2 -h-deg. so that a solar heat absorption of 750 kcal/m.sup.2 -h and a heat-delivering surface area F = 2.5 m.sup.2 result in a maximum temperature of 750/2.2 .times. 2.5 = 130.degree. C. When it is realized that in accordance with published data a heat insulation in a thickness up to 15 cm is required for such .DELTA.T, it will be appreciated that the efficiency cannot be increased further in this way although such increase would be desirable for technical and economic reasons.